Recent experiments in our laboratory have shown that certain charged cyanine dyes absorb into bilayer lipid membranes and induce transmembrane photo-voltages by their re-orientation within the membrane as a result of photo-excitation. Variations in the photo-voltage waveforms brought about by changes in dye structure and membrane composition indicate that these dyes can provide sensitive measurements of the structure and intermolecular organization of biological membranes. The objective of the proposed research is to investigate cyanine dyes for use as photo-electric probes of membranes, using bilayer lipid membranes and vesicle suspensions. Selected dyes will then be used as probes to study the detailed structure and intermolecular organization of the cell membranes which seem germane to current investigations of human cancer.